The invention relates to a toner containing magnetite particles.
Particulate magnetites that can be produced from aqueous solutions by a precipitation process have been known for a long time. U.S. Pat. No. 802,928 already describes the production of magnetite by precipitating iron(II) sulfate with an alkaline component and subsequent oxidation with air. Numerous, further subsequent documents likewise describe the production of magnetites by the precipitation process.
Pure precipitation magnetites without traces of other metals or elements can be produced batchwise according to DE-A 3,209,469 or continuously according to DE-A 2,618,058.
Normally, FeSO4 is used as iron(II) salt. It is, however, also possible to use any soluble iron(II) salt to produce a magnetite by the precipitation process. In particular, FeCl2 is suitable as described in DE-A 3,004,718. FeSO4 or FeCL2 have the advantage that both substances can be obtained in large quantities very inexpensively as waste materials from the iron-processing industry.
In addition to the most frequently used sodium hydroxide, CaO or CaCO3 (DE-A 3,004,718), ammonia (DE-A 2,460,493) or Na2CO3, MgCO3 or MgO (EP-A 0,187,331) are also suitable as precipitating agent. Usually, air is used as oxidizing agent. However, processes for oxidation with nitrates (DD-A 216,040 and DD-A 284,478) are also described.
Magnetites were first used to produce all kinds of paints. The particular advantage of magnetites over organic dyestuffs and soot is their much better weather resistance. Paints containing magnetite can also to be used outdoors.
Furthermore, precipitation magnetites are favored for use in coloring concrete moldings, such as, for example, concrete paving stones or concrete roof tiles.
Magnetites have also been used for some time in electrophotography for producing toners. Magnetites that have been produced by the precipitation process are preferably used to produce toners for copying machines using single-component toners. The magnetic toners used for this purpose must have various properties. With the progressive development and improvement of copying machines and printers, the requirements relating to magnetic toners and, consequently, to the magnetite used for this purpose have become increasingly greater. The latest printer generation achieves a resolution of more than 400 dpi (dots per inch), for which purpose it was necessary to develop finely divided toners having very narrow particle size distribution. This had the result that the magnetites used for this purpose likewise have to have very narrow particle size distribution. Furthermore, a certain particle size is necessary to ensure a homogeneous distribution of the magnetite particles in the finished toner. The magnetites themselves must have a sufficiently high electrical resistance to stabilize the latent image during the electrostatic transfer. Furthermore, coercive force, saturation magnetization and, in particular, the remnant magnetization must be in the correct relationship to the field strengths prevailing in the machine.
Si-containing magnetites are used for the application in magnetic toners. These have a different charge behavior to that of pure magnetites and have a higher thermal stability for the same particle size. A process for producing such particles is described in JP-A-61 034 070. In this case, the Si component is added to the iron(II) sulfate, but this results in precipitations of silicic acid and, consequently, in nonuniform distribution of silicon in the magnetite lattice. The production of magnetites by the precipitation process using silicon is also described in JP-A-51 044 298. U.S. Pat. No. 4,992,191 describes a magnetite containing 0.1 to 5.0 atomic % of Si relative to Fe that is claimed to be particularly suitable for producing toners. In the process described therein, a silicate component is added to an alkaline component in the form of an aqueous solution and then an iron(II) component in the form of an aqueous solution in an amount that is such that the molar ratio of Fe(II) component to alkaline component is roughly       0.53    ⁡          [              1.5        2.85            ]        ,
the temperature being kept at 90xc2x0 C. The suspension obtained in this way is then treated with air as oxidizing agent in order to obtain spheroidal, silicon-containing magnetite having a particle size in the range from 0.1 to 1.0 xcexcm. The particles obtained are filtered, washed and ground.
DE-A 19 702 431 describes a further process for producing particularly round Si-containing magnetites. In the latter, the production of low-silicon, almost Si-free, round magnetites is also described in detail for the first time. The thermal stability of said magnetites is, however, described as inadequate for the production of magnetic toners, with the result that, according to this teaching, the presence of silicon was regarded as necessary to produce magnetites suitable for producing toners.
An object of the present invention was to provide a particularly inexpensive process for producing magnetites having properties suitable for the production of magnetic toners and also the toners producible with said magnetites.
Surprisingly, it was found that the magnetites, described in DE-A 19 702 431 as insufficiently thermally stable, can be used very satisfactorily in toners. In addition, the production process is less expensive as the result of the elimination of the Si-introducing component, which also makes one process step unnecessary.
The invention relates to a toner containing low-silicon magnetites.